1. Field of the Invention
The present invention relates generally to an ignition distributor, particularly to an ignition distributor in which an electric wave noise caused by an electric discharge between rotary and stationary electrodes is minimized.
2. Description of the Related Art
There is conventionally a known ignition distributor having a plurality of stationary electrodes electrically connected to ignition plugs, respectively, and a rotary electrode rotating with a minute minimum distance maintained between one end thereof and the stationary electrodes and with a voltage application to the other end thereof, an electric discharge being triggered to occur between a discharge element of the stationary electrode and a discharge element composing the one end of the rotary electrode, thereby distributing electric power to the ignition plugs alternately when the rotation of the rotary electrode brings the discharge element of the rotary electrode alternately into face of the discharge elements of the stationary elements substantially at the minute minimum distance therebetween.
The conventional ignition distributor suffers from electric wave noise because of spark discharge for distribution, affecting radios, televisions and other telecommunication devices. To solve this problem, Japanese Unexamined Patent Publication (Kokai) No. 1-262368 proposed an ignition distributor having a discharge element composed of a ferrite core surrounded by a surface layer containing silicon oxide, having a thickness of from 10 to 100 .mu.m and many concavities and protrusions with a diameter of from 0.5 to 50 .mu.m.
The proposed ignition distributor is very advantageous for suppressing a radiated electric wave noise but not radio noise in a weak electric field area distant from broadcasting stations for the following reason.
The electric discharge across a minute gap between stationary and rotary electrodes of an ignition distributor includes a capacitance discharge owing to a dielectric breakdown of the gap and an induced discharge along a low resistivity zone in the gap that is generated by ionization owing to the capacitance discharge.
The capacitance discharge occurs over a short time, has a high voltage and a large current and usually generates a strong electric wave noise. The induced discharge occurs over a long time, has a low voltage and a small current and is generally said to contribute to electric wave noise only to a very small extent. Accordingly, the conventional study focuses on electric wave noise caused by the capacitance discharge.
The electric wave noise owing to an electric discharge is regulated by law in Europe, the U.S.A., and some other countries. Electric wave noises are classified into two cases; the first in which electric wave noise radiated from automobiles, for example, affects external radios, televisions and other telecommunication devices (hereinafter referred to as "radiated electric wave noise") and the second in which high frequency electric wave noise affects a car radio or other electronic devices mounted on a car via the antenna, the electric wiring, the car body or other wave paths (hereinafter referred to as "FM radio noise").
Nevertheless, FM radio noise was conventionally considered to be related to radiated electric wave noise, and accordingly, the noise suppression effect was estimated only for the radiated electric wave noise by using an estimation method specified in Europe and the U.S.A. under regulation by law.
The present inventors made a detailed study on the influences brought about by radiated electric wave noise and FM radio noise, separately.
Research was undertaken on the relationship between the estimated value (i.e., radiated electric wave noise) obtained by using the method specified in Europe and the U.S.A. and actual FM radio noise both in strong electric field areas near broadcasting stations and in weak electric field areas distant from broadcasting stations. The result shows that a good relationship is observed in the strong electric field areas such that FM radio noise was low when the estimated value was small, but in weak electric field areas, the relationship is poor, and on one occasion, FM radio noise did occur when the estimated value was small, and on another occasion, no FM radio noise occurred when the estimated value was large.
This is considered because improvements in conventional ignition distributors has significantly reduced the substantial influence of the electric wave noise caused by a capacitance discharge both on the outside and the inside of a car, and thereby, the influence of the noise caused by an induced discharge, concealed behind the influence of a capacitance discharge noise, has been revealed particularly in the case of a car radio having the receiving port located very close to the noise source.